Article pubs.acs.org/Macromolecules
Radiation-Induced Inclusion Polymerization of Acrylonitrile in Urea Canals: Toward Synthesis of Completely Isotactic Polyacrylonitrile with Controlled Molecular Weight Jun-Ting Zou, Yu-Song Wang, Wen-Min Pang, Lei Shi,* and Fei Lu* Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China ABSTRACT: Completely isotactic polyacrylonitrile (meso/meso triad >99%) has been synthesized successfully by radiationinduced inclusion polymerization of acrylonitrile in urea canals. The long-lived nature of the growing radical species and the linear dependence of molecular weight on conversion were observed. The molecular weight distribution of product polymers was relatively narrow (99%) has not yet been obtained. The simultaneous control of the molecular weight and the tacticity for the polymerization of AN has not been attained. In present work, we report the synthesis of completely isotactic polyacrylonitrile (i-PAN) by radiation-induced inclusion polymerization of AN in urea canals. The growing radical species show the long-lived nature, and the linear dependence of molecular weight on conversion is observed. Moreover, the MWD of i-PANs is relatively narrow (99 >99 >99 >99 >99 85 62 >99 >99 >99 >99 >99
a
Because of the existence of termination and chain transfer reactions, the G-value calculated may have no meaning. 1766
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Figure 4. HSQC spectrum of i-PAN.
triads36 and heptads37 are shown in Figure 3. Viewed from triads, it can be reasonably inferred that the single peak in the region of CN carbon of i-PAN should be assigned to the (mm) sequence. Additionally, according to the assignment with heptads given by Wang et al.,37 in the nitrile carbon spectra of i-PAN and a-PAN, the peak at 120.41 ppm should be assigned to the (mmmmmm) sequence or even longer isotactic sequence of PAN. For i-PAN, the single peak at 120.41 ppm indicates that there is only one isotactic configuration along the whole chain. Prudently considering the detection limit of 13C NMR, the triad isotacticity (mm) is more than 99%. Figure 5 shows the XRD patterns of i-PAN and a-PAN. It is found that when the isotacticity is increased, the main peak at
Figure 2. Schematic representation of the inclusion polymerization. The recovery step is omitted.
step, isotactic chains propagate within the urea tunnels at −110 to −90 °C. The well-organized confine media leads to not only a highly stereospecific propagation but also a long-lived nature of the growing radical species by suppressing the bimolecular termination and the chain transfer reactions. Moreover, since the initiation (γ-ray irradiation step) and the chain propagation are separated, all chains are essentially propagating at the same time, which consequently leads to the synthesis of polymers with predetermined molecular weights and low MWD. After the chain propagation step, the urea tunnels are dissolved, and the chain propagation is terminated. 3.2. Isotacticity of Prepared PAN. The typical 13C NMR spectra of i-PAN and a-PAN, and the HSQC spectrum of iPAN are shown in Figure 3 and Figure 4, respectively. The differences in stereoregularity appear in the nitrile (CN) carbon and methine (CH) carbon in the 13C NMR spectra. Single peak appears in the region of CN carbon, CH2 carbon, and CH carbon in the 13C NMR spectrum of i-PAN, which indicates that there is only one stereoconfiguration along the chain direction. Assignments of the PAN nitrile carbon spectra with
Figure 5. XRD patterns of i-PAN and a-PAN.
about 17 degree (2θ) is sharpened and shifted from 17.1 to 16.77°. As revealed from Figure 5, the intermolecular distance is 5.2822 Å and the half-value width of the main peak is 0.96°, which match the results predicted (intermolecular distance 5.275 Å, half-value width of the main peak 0.97°)38 and reveal that the PAN sample is a completely isotactic PAN (mm >99%). 3.3. Characteristics of the Inclusion Polymerization. Theoretically, the walls formed by urea molecules can prevent bimolecular termination between growing polymer chains unless two growing polymer chains are both present in a urea canal. This implies the long-lived nature of the radicals in the urea tunnels in inclusion polymerization. The typical ESR spectrum measured at −110 °C of the AN/UIC after γ-ray
Figure 3. 100-MHz 13C NMR spectra of i-PAN and a-PAN samples. 1767
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and consequently the growing radical species show the longlived nature. Namely, inclusion polymerization possibly enable not only tacticity control but also molecular weight control without using additional reagents for the living radical polymerization. Figure 8 shows the plots of the conversion and isotacticity vs. polymerization time and the dependences of Mn and Mw/Mn on conversion. The linear dependence of Mn on conversion is observed at the early stage of polymerization (before ∼35% conversion). Evidently, the dual control of the molecular weight and the tacticity for the inclusion polymerization of AN is achieved. After ∼35% conversion, with the growth of polymer chains and the consumption of AN molecular arrays, bimolecular termination takes place between two growing polymer chains in the same urea canal, and consequently the relationship of molecular weights between the conversion deviates from linearity at high conversion. As can be seen from Figure 8, the AN/urea molar feed ratio affects the conversion. The conversion is essentially dominated by the initiation and the chain propagation. The γ-ray irradiation at liquid nitrogen temperature is the initiation step. High energy radiation can penetrate into the crystalline lattice of inclusion compounds, and AN monomers in urea tunnels absorb radiation energy to produce free radicals. Urea lattice can also absorb radiation energy, and consequently the AN/urea molar ratio will affect the initiating efficiency. As a measure of the initiating efficiency, the yield of radicals generated by the absorption of a given radiation dose is the most important characteristic of the initiation step. This characteristic is defined by the G-value or 100-eV yield of initiating radicals.45 G-value is calculated according to the formula:45
irradiation is shown in Figure 6a. The ESR spectrum looks too complex to be assigned. Since both AN and urea in AN/UIC
Figure 6. ESR spectra of AN/UIC measured at −110 °C after γ-ray irradiation (a), and the simulated spectrum (b). Conditions: AN/urea mole ratio, 1:3; aging time, 16 days; radiation dose, 18 kGy.
can absorb radiation energy to produce free radicals,39−41 we hypothesized that the spectrum is corresponding to the superposition of two different species, i.e. the acrylonitrile propagating radical (APR) ∼CH2Ċ HCN and the urea radical (UR) H2NCOṄ H. Coupling constants and g-values for APR42 are A1[1H] = 22.22 G, A2[1H] = 20.19 G, A[14N] = 3.48 G, and g3 = 2.0030 and those for UR43 are A1[1H] = 15 G, A2[1H] = 23 G, A3[1H] = 31 G, A1[14N] = A3[14N] = 0 G, A2[14N] = 40 G, g2 = 2.00, and g1 = g3 = 2.0061, respectively. The contributions of APR and UR to the original ESR spectrum of AN/UIC can be simulated by the software WinSim.44 Using coupling constants and g-values for these two radicals, one can obtain the simulated spectrum as shown in Figure 6b. The simulation shows a good agreement with the observed spectrum (Figure 6a) with by 14.21% APR plus 85.79% UR. Figure 7a shows the
G(R•) =
MPAN × Na f × D × Mn
(1)
where G(R•) is the radiation chemical yield expressed in initiating radicals/100 eV. MPAN is the mass in g of the product polymers, Na is Avogadro’s number, Mn is the number-average molecular weight of the product polymers, D is the total dose in kGy, and f is the conversion factor for the conversion of kGy to 100 eV (6.19 × 1016). As seen from Table 1, the G value is in the range 0.78 to 1.24 for the AN/urea ratio from 1/5 to 1/1.2, which is much lower than that of pure AN at 20 °C (i.e., 5.0).45 The MWD (Mw/Mn) of i-PANs is no more than 1.5 and even less than 1.4 at low conversion (Figure 8). The MWD describes the uniformity of chain growth. Polymer chains propagate in urea tunnels, and the length of chains depends on the length of AN molecular arrays. The location of radicals initiated by γ-rays is completely random. With the growth of polymer chains and the consumption of AN molecular arrays, the MWD will increase slightly. 3.4. Essential Factors Obtaining i-PAN. 3.4.1. AN/UIC Formation Factors. The formation process of AN/UIC has been discussed in previous research.46 AN molecules enter urea lattice gradually, depending on the aging time, and once AN molecules enter urea lattice, AN/UIC possesses the final structure. With sufficient AN molecules, the length of AN molecular arrays in urea canals increases as aging time prolonging until urea tunnels are saturated by AN. When the tunnels of urea lattice are occupied completely by AN molecules, the guest/host ratio of AN/UIC is provided as 1.17.46 Provided that AN/urea molar feed ratio is no more than 1.17/1, AN monomers will be included in urea canals totally
Figure 7. ESR spectra of the AN/UIC measured at different time points after γ-ray irradiation (a) and the double integral of the corresponding ESR spectra (b). The sample was stored at −110 °C, and tested by ESR at the same temperature. Three spectra were measured in the same condition. Conditions: AN/urea mole ratio, 1:3; aging time, 16 days; radiation dose, 18 kGy.
ESR spectra of the AN/UIC measured at different times after γray irradiation and Figure 7b shows the double integral of the corresponding ESR spectra. The sample was stored at −110 °C and tested for ESR at the same temperature. The double integral of ESR signals measured at different times does not change significantly. This suggests that the walls formed by urea molecules can prevent termination and chain transfer reactions, 1768
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Figure 8. Conversion and isotacticity in polymerization time (a) and dependence of Mn and Mw/Mn on conversion (b). Radiation dose: 24 kGy. Polymerization temperature: −100 °C.
3.4.2. γ-Ray Irradiation Factors. Since an ideal isotactic configuration can be attained only within the urea tunnels, keeping the intactness of the tunnels during the whole inclusion polymerization is the key to obtaining i-PAN. At the γ-ray irradiation step, besides radicals, the heat can also be produced by γ-ray irradiation. The local temperature may be elevated by the heat converted by the radiation energy, and the inclusion compound will be destroyed, as a result, the isotacticity of PANs will be decreased. Therefore, the local temperature elevation cannot be overlooked so as to obtain i-PAN. The irradiation dose rate affects the rate of initiating radical formation, and consequently the yield of prepared PAN, but hardly affects the isotacticity of PAN (Table 1). 3.4.3. Chain Propagation Factors. At the chain propagation step, radiation-induced radicals propagate to give polymer chains. The heat of polymerization of AN is 77 kJ/mol,48 which is much larger than that of the decomposition of the AN/UIC (5361.53 J·mol−1).46 This means that the heat of polymerization needs to be controlled. It is known that the polymerization rate increases with the increasing temperature, and the higher reaction rate means the higher efficiency and faster generation of heat produced. It will lead to a rise in temperature, and hence to a vicious circle, destroying inclusion compounds and reducing the isotacticity of PANs. Thus, the appropriate polymerization temperature needs to be considered to control the polymerization rate and the heat generation. Figure 10 shows the effect of polymerization temperature on the isotacticity of PANs. Only when the polymerization temperature is lower than −90 °C, i-PAN can be obtained. On the other hand, even if the polymerization temperature is lower than −90 °C, since the thermal conductivities of the urea and PAN are poor, the heat transfer of the solid-state chemical reactions needs to be improved. The local temperature elevation by the heat of polymerization will destroy inclusion compounds, decreasing the isotacticity of PANs. The self-made reactor for the chain propagation step (Figure 1) is designed to remove the heat of polymerization quickly and effectively. In order to evaluate the effect of the heat transfer at the chain propagation step on the isotacticity of PANs, the nitrogen was turned off in some experiments and consequently the irradiated AN/UIC was packed on the sintered disk rather than suspended directly by the flow of the nitrogen (Figure 11a). In this case, the molecular weight (Mn) of product polymers is no longer a linear function of the conversion and the MWD
after a sufficient aging time at low temperature (−78 °C to −55 °C). If the AN/UIC is irradiated by γ-rays before the formation process is completed, besides AN monomers in urea canals, those outside the tunnels (free AN) can also be initiated by γrays, which will be polymerized without specific stereoregularity, reducing the isotacticity of product polymers. The presence of free AN can be confirmed by DSC curves, from which two peaks at around −80 and −30 °C, respectively, can be found. The former is the melting peak of AN and the latter one is the endothermic peak caused by the decomposition of AN/UIC.47 Figure 9 shows the DSC curves of AN/UIC
Figure 9. Effect of aging time at the AN/UIC formation step on NMR results of PANs. Conditions: AN/urea mole ratio, 1:1.2; aging temperature, −60 °C; radiation dose, 24 kGy; polymerization temperature, −100 °C; polymerization time, 4 h.
prepared with different aging times and the effect of aging time at the AN/UIC formation step on NMR results of PANs. The isotacticity of PANs measured as a function of aging time shows a continuous change, which demonstrates that to obtain completely isotactic PAN (i-PAN), AN monomers should be included in urea canals totally before γ-ray irradiation, leaving no free AN outside the tunnels. 1769
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isotactic polyacrylonitrile, the following conditions should be met simultaneously: (1) AN monomers are included in urea canals totally before γ-ray irradiation, (2) the polymerization temperature should be lower than −90 °C, and (3) the heat of polymerization is removed effectively.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: (L.S.)
[email protected]; (F.L.)
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the Fundamental Research Funds for the Central Universities (No. WK2340000016), and USTC Innovation Funds for Graduate Students (No. KD2008082).
Figure 10. Effect of polymerization temperature on the isotacticity of PAN. Conditions: AN/urea mole ratio, 1:2; aging time, 16 days; radiation dose, 24 kGy; polymerization time, 5 h.
increases to 2.1 (Figure 11b), which indicates the occurrence of chain transfer reactions. That is, the local temperature elevation by the heat of polymerization destroys inclusion compounds, and propagating radicals are no longer encapsulated, and naturally chain transfer reactions occurs, decreasing the isotacticity of PANs. It follows that the heat transfer at the chain propagation step has an important influence on the isotacticity of PANs. Evidently, in order to obtain i-PAN, it is both important and necessary to remove the heat of polymerization effectively.
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REFERENCES
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4. CONCLUSIONS In this study, i-PAN has been synthesized successfully by radiation-induced urea inclusion polymerization (inclusion polymerization) and the characteristics of the inclusion polymerization are described in detail. The growing radical species show the long-lived nature since the walls formed by urea molecules can prevent termination and chain transfer reactions. The linear dependence of Mn on conversion is observed, and the molecular weight distribution of i-PANs is relatively narrow (
